Water Mass Transports and Pathways in the North Brazil‐Equatorial Undercurrent Retroflection

• Published in: Journal of geophysical research. Oceans 2022-05, Vol.127 (5), p.n/a
• Main Authors: Vallès‐Casanova, Ignasi, Fraile‐Nuez, Eugenio, Martín‐Rey, Marta, Sebille, Erik, Cabré, Anna, Olivé‐Abelló, Anna, Pelegrí, Josep L.
• Format: Article
• Language: English
• Subjects: Atlantic Meridional Overturning Circulation (AMOC); Brazil Current; Density; Equator; Equatorial regions; Equatorial undercurrents; Geophysics; Hemispheres; Interannual variability; Lagrangian transports; Mixed layer depth; Oceans; Pycnocline; Retroreflection; Seasonal variations; Seasonality; Spring; Thermocline; Trade winds; Transport; Tropical Atlantic; Tropical climate; Tropical environments; Undercurrents; Variability; Water; Water circulation; Water masses; Water sources; Water transport; western boundary current
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2021JC018150

The equatorial retroflection of the North Brazil Current (NBC) into the Equatorial Undercurrent (EUC) and its posterior tropical recirculation is a major regulator for the returning limb of the Atlantic Meridional Overturning Circulation. Indeed, most surface and thermocline NBC waters retroflect at the equator all the way into the central and eastern Atlantic Ocean, before they recirculate back through the tropics to the western boundary. Here, we use cruise data in the western equatorial Atlantic during April 2010 and reanalysis time series for the equatorial and tropical waters in both hemispheres in order to explore the recirculation pathways and transport variability. During the 1998–2016 period, the annual‐mean EUC transports 15.1 ± 1.3 Sv at 32°W, with 2.8 ± 0.4 Sv from the North Atlantic and 11.4 ± 1.3 Sv from the South Atlantic. At 32°W most of the total EUC transport comes from the western boundary retroflection south of 3°N (7.2 ± 0.9 Sv), a substantial fraction retroflects north of 3°N (5.6 ± 0.4 Sv), and the remaining flow (2.3 Sv) joins through the interior basin. The South Atlantic subtropical waters feed the EUC at all thermocline depths while the North Atlantic and South Atlantic tropical waters do so at the surface and upper‐thermocline levels. The EUC transport at 32°W has a pronounced seasonality, with spring and fall maxima and a range of 8.8 Sv. The 18 yr of reanalysis data shows a weak yet significant correlation with an Atlantic Niño index, and also suggests an enhanced contribution from the South Atlantic tropical waters during 2008–2016 as compared with 1997–2007. Plain Language Summary In the western margin of the tropical Atlantic Ocean, the North Brazil Current (NBC) carries warm and salty waters across the equator northward into the Caribbean Sea. However, an important fraction turns eastward in the equatorial region, retroflecting into the Equatorial Undercurrent (EUC). We find that indeed most of the NBC‐EUC retroflection water comes from the tropical and subtropical South Atlantic Ocean, but we observe that the EUC is also fed by North Atlantic waters, mostly from the north‐eastern tropical Atlantic. Remarkably, these northern and southern water sources follow different retroflection pathways at different latitudinal bands and density layers. Subtropical water from the South Atlantic is present in all density layers while other sources reach the EUC at shallower layers. We also observe that the water transport associated wi